1. Field of the Invention
This invention relates to lithographic apparatus and methods.
2. Background Information
The term “patterning device” as here employed should be broadly interpreted as referring to structure that may be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The terms “light valve” and “spatial light modulator” may also be used in this context. Generally, such a pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such a patterning device include:                A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam may cause selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.        A programmable mirror array. One example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of very small (possibly microscopic) mirrors, each of which may be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuation means. The mirrors may be matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction with respect to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronic circuitry. In both of the situations described hereabove, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which are incorporated herein by reference, and PCT patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, the support structure for the array may be embodied, for example, as a frame or table which is fixed or movable as required.        A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied, for example, as a frame or table which is fixed or movable as required.        
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask (or “reticle”) and mask table; however, the general principles discussed in such instances should be seen in the broad context of the patterning device as hereabove set forth.
Lithographic projection apparatus may be used, for example, in the manufacture of devices such as integrated circuits (ICs). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (e.g. a wafer of silicon or other semiconductor material) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a network of adjacent target portions that are successively irradiated via the projection system (e.g. one at a time).
Among current apparatus that employ patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus—commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Since, in general, the projection system will have a magnification factor M (generally<1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. A projection beam in a scanning type of apparatus may have the form of a slit (e.g. rectangular or arcuate) with a slit width in the scanning direction. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, which is incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will be repeated for each new layer.
Eventually, an array of devices will be present on the substrate (wafer). The devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing,” Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
The term “projection system” should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. For the sake of simplicity, the projection system may hereinafter be referred to as the “lens.” The radiation system may also include components operating according to any of these design types for directing, shaping, reducing, enlarging, patterning, and/or otherwise controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens.”
Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices, the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and PCT Application WO 98/40791, which documents are incorporated herein by reference.
In a lithographic projection apparatus, if a substrate table is moved by an actuator attached to a base frame, a reaction force is transferred to the base frame. In some apparatus, a substrate table may weigh as much as 40 to 70 kg or more, and the acceleration of the substrate table may be on the order of 5 to 60 m/s2. Therefore, the reaction force may be high enough to cause deleterious vibrations or movement of the base frame.
The effect of such a reaction force can be reduced by the use of a freely mounted balance mass. For example, a positioning actuator may be used between the substrate table and the balance mass such that the substrate table is moved in one direction whilst the balance mass is moved by the reaction force in the opposite direction. In such case, the balance mass may be moved by an amount equal to the distance moved by the substrate table times the mass ratio of the substrate table to the balance mass. In this way, no substantial reaction forces are transmitted to the base frame, and the centre of gravity of the balance mass and the substrate table relative to the base frame stays relatively constant.
The balance mass and substrate table are mounted, as near as possible, in a frictionless manner, e.g. using air bearings, such that no further parasitic forces are transmitted to the base frame or other parts of the lithographic projection apparatus as a result of movement of the substrate table.
To meet the ever-present demand for imaging features of reduced size, it is desirable to reduce the wavelength of radiation used for the projection beam. Thus, the need for improved resolution encourages the development of lithographic projection apparatus making use of extreme ultraviolet radiation (EUV) (e.g. with a wavelength in the range of 5–20 nm). Currently, an EUV lithographic projection apparatus must use mirrors in the projection system because no material suitable for forming refractive optical elements for EUV is yet known.
Additionally, the beam must be kept in vacuum to avoid contamination and attenuation of the beam. Unfortunately, the requirement for the projection beam to be kept in a vacuum means that the use of air bearings for the balance mass is no longer practical. Although a design has been proposed in which the air bearings are covered by a metal bellows, this design is complicated and heavy.